The basic structure of an organic light emitting diode (OLED) consists of a stack of thin layers of organic material sandwiched between a transparent anode and a metallic cathode. The organic material layers comprise a hole-injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer. When an appropriate voltage (typically between 2 and 30 volts) is applied to the device, the injected charge excites the chromophores to produce light (electroluminescence) by the radiative recombination of injected positive and negative charges in the emissive layer. The structure of the organic layers and the anode and cathode are chosen to maximize the radiative recombination process in the emissive layer, thus maximizing the light output from the OLED device. Similar devices are polymer light emitting diodes (PLEDs). An alternative source of energy to excite the chromophores is a light source, such as a laser generating optical pulses at a wavelength and intensity sufficient to cause the light emitter layer to emit light, generally in the range of 150 nm to 2000 nm.
Chromophore, lumophore, fluorophore, light emitting material are synonyms for compounds emitting radiation with optical frequencies upon their excitation from external energy sources. Radiative recombination (radiative decay, radiative relaxation) refers to the direct transition of a chromophore from excited to ground state accompanied by emission of a light quantum (photon).
Triplet emitters, i.e., phosphorescent materials with low oscillator strength of transition responsible for the emission of luminescence caused by the triplet nature of the excited state, are useful materials in such photonic applications. Indeed, the triplet nature of the emitting state ensures that in light emitting devices, such as OLEDs or PLEDs, the 25% efficiency limit predicted previously for singlet emitters does not exist [Patel et al.]. However, such materials typically exhibit small oscillator strengths of the radiative transition and very long emission life-times (up to several seconds). This leads to low luminescence yields due to competition with faster non-radiative processes. Also, slow radiative recombination limits the performance of LEDs due saturation of lumophores and non-radiative processes such as triplet-triplet quenching, at high rates of carriers injection.
Thus, it is would be very useful to be able to accelerate the radiative recombination processes in triplet emitters while keeping other optical properties unchanged. Routes for the radiative decay manipulation are found in the formula for the Einstein coefficient A21, governing the spontaneous emission of two-level system [Snoeks et al.]; see equation 1:A=πω|D|2ρ(r, ω)/∈(r)≡krad  (1)where ω is an emission photon cyclic frequency, D—matrix element of transition dipole, ∈—dielectric constant of surrounding medium, and ρ is a local density of states (LDOS) for the electromagnetic field. As seen from equation (1), the rate of radiative decay can be manipulated by means of variation of LDOS and/or dielectric constant of environment. The latter parameter varies only slightly in optically transparent materials suitable for LED applications, allowing very little control over krad. In contrast, variation of LDOS could be very large if the light emitting material is placed in a specially designed environment [Lakowicz; Philpott et al.; Weitz et al.].
It is known from previous reports, that LDOS can be increased in the vicinity of metal structures supporting surface plasmons [Lakowicz]. Surface plasmons (surface plasmon polaritons) are collective two dimensional oscillations of electron density. To achieve optimal performance from the chromophore, one has to balance several dynamical processes. First, the radiative decay can be accelerated in the vicinity of metal surfaces, due to modification of the local density of states from the electromagnetic field of the surface metal [Weitz et al.]. Second, one can expect an increase of the absorption cross-section of the material because of the incident field enhancement by the surface plasmon [Lakowicz; Weitz et al.; Raether]. Third, the chromophore's interactions with metal surfaces can introduce additional non-radiative losses caused by Förster-type energy transfer between the emitter and a metal [Lakowicz; Weissberger et al.]. The latter process leads to quenching of light emission, and could significantly degrade the luminescence yield. This non-radiative channel of the energy relaxation can be eliminated if the separation between the light emitting species and the metal surface is large enough. The rate of dipole-dipole Förster energy transfer follows the 1/r6 law [Förster]. Spatial decay of the transfer rate depends on the system geometry, for bulk metals it could be 1/r3 and for large metal particles −1/r4. See equation 2:                               k          F                =                              1                          r              6                                ⁢                      1                          τ              rad                                ⁢                      ∫                                                            A                  D                                ⁡                                  (                  λ                  )                                            ⁢                                                I                  A                                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                                        (        2        )            where r is the distance between the chromophore and the metal surface, AD(λ) and IA(λ) are absorption and emission spectra of the metal and chromophore, respectively, and λ is the wavelength. It follows from equation (2) that the Förster energy transfer mechanism is efficient only within a limited volume around acceptor species (i.e. metal in our case). It is possible to introduce a spacer with a critical length Lc, such that at distances larger than Lc energy transfer process is negligibly small. Thus, one could achieve a net enhancement of the lumophores emissive properties if the enhancement (Enhancement=krad/krad0—, krad0 is the radiative decay rate of isolated molecule) is non-zero at distances larger than Lc. See FIG. 1 which shows the typical distance-dependence for the Förster-type emission quenching rate for triplet (dashed curve) and singlet (dotted curve) emitting chromophores in the vicinity of a metal particle (Au 20 nm diameter). Circles correspond to the distance-dependence of emission rate enhancement calculated for the same conditions using the Mie scattering approach [Chew]. For triplet emitting chromophores with life-times of about 1 microsecond, the net enhancement is observed if chromophore/metal separation is larger than Lc. For singlet emitting chromophores, with high oscillator strength, efficient energy transfer makes the enhancement difficult.
Thus far, there are no reports on radiative decay control in triplet emitters based on interactions with surface plasmons. Several theoretical papers consider issues of the radiative life-time modification in the vicinity of metal planar surfaces [Raether] and nanoparticles [Weitz et al.; Chew]. The Förster-type emission quenching has been studied in several papers (see, e.g. [He et al.]) but primarily as a separate process. Several attempts have been made to alter the decay rate in singlet emitters [Lakowicz], but were inconclusive since acceleration of the excited state decay in this case could be attributed to fast quenching processes. The latter may be very efficient for singlet chromophores because of their large transition moment and short intrinsic radiative lifetime (see equation (2)). Surface plasmon-light interactions are covered in numerous journal papers, books, and reviews (e.g. [Raether]) mostly in conjunction with optical surface enhanced phenomena, such as Raman scattering [Vo-Dinh], second harmonic generation [Lue et al.], two-photon absorption [Gryczynski et al.], etc. There are no reports on surface plasmon-enhancement of organic triplet emitter-based LEDs.